Jug kettles used to boil water for the making of instant coffee or tea have been around for many years. The jug is filled with tap water, the kettle is connected to the electrical cord or base (which is permanently connected to the mains) and the switch is turned on. When the water starts to boil, steam will trip a bimetallic temperature switch turning off the electrical element at the bottom of the kettle. The water is now ready to be poured.
There are some variants of this design some of which are common to most kettles. The use of a flat electrical heating element at the bottom of the jug is a popular design. This makes cleaning lime scale build-up easier.
In another variant, the element is switched off by the use of a bimetallic strip in contact with the bottom of the kettle and not through the use of vapour.
There is also the variant where the temperature at which the electrical element is switched off is controllable through the use of a tuning knob on the bimetallic strip. This allows the water to be heated to below 100 C which can reduce scalding. Many of these types of devices are familiar to users and will not be described in any more detail.
Conventional kettles have got two main problems. Firstly, if the kettle is filled to the top, it will take a long time, around 3 or 4 minutes, for the water to boil. Often, the only requirement is however for one or two cups of boiling water. However if the kettle is only filled a little bit from the bottom (enough for a cup or two), there is a constant requirement to be re-filing the kettle. It is also difficult to judge the required amount of water while filling.
The second problem is the lime scale that precipitates out of hard tap water as the water is heated. This deposits on the kettle requiring either cleaning or in self cleaning kettles the precipitate will wash out in larger white scales that can drop out at the bottom of the kettle with the last water. The lime scale will also produce a scum on the coffee or tea making the drink look unpleasant. The removal of other types of pollutants in the tap water can also be important.
There are on the market water filters and even water filters combined with a kettle have been recently introduced (for example from Brita). These however, working by gravity, take quite a while for the water to filter through and do not provide a method of positively displacing the water required to be heated.